If I understand you, you are saying that when you apply pressure to kick start, the kicker will slip down as though the gears are out of alignment. Correct? If so, you have already answered your question. When you engage to kick start, you should be able to feel contact "gear meshing", which means the shaft is applying load to the engine. If this is only momentary and the gear slips freely when you apply force, your gears are worn or you have some bushings that are worn and are creating a misalignment. This is not difficult to troubleshoot and identify the root cause and not hard at all to fix.